This invention relates to an apparatus for producing a magnetic field in a space between two oppositely arranged permanent magnets, which is suitable for use in magnetic resonance imaging (MRI).
For producing a magnetic field for MRI, a conventional apparatus uses a pair of permanent magnet discs which are held at a suitable distance from each other by a yoke such that a pole face of one magnet disc faces the opposite pole face of the other. To produce a uniform magnetic field in a cylindrical space between the two permanent magnet discs, a disc-like pole piece is placed on the aforementioned pole face of each permanent magnet disc. The pole piece is made of a ferromagnetic material such as soft iron and has an annular protuberance in a peripheral region.
For practical applications of the above described magnet apparatus it is desirable to reduce the size and weight of the whole apparatus, and to meet the desire it is effective to reduce the size and weight of the yoke. However, there is a problem that as the size and weight of the yoke are substantially reduced it is likely that the uniformity of the magnetic field in the apparatus is degraded by the appearance of leakage flux which is attributed to magnetic saturation of the yoke. When much care is taken to prevent the occurrence of magnetic saturation it becomes difficult to considerably reduce the size and weight of the yoke.